The objective of this proposed research is to study nuclear function and the regulation of gene expression in Paramecium aurelia and is directed toward the use of Paramecium as a model system in the study of cellular aging and differentiation. Our approach is two-fold: 1) to isolate specific genes such as rDNA, histone and surface antigen and make direct comparison of sequence divergence between closely related species as well as with aging cell lines and 2) to take advantage of the ability to microinject Paramecium with well defined cell components. It will be particularly interesting to compare the effect of cytoplasmic components on the aging process as well as to determine whether surface antigen message can function in a cell grown at an otherwise non-permissive temperature.